The metallic materials of industrial cooling and/or heat-transfer systems are subject to more or less strong chemical and mechanical stresses when used with aqueous, glycol-containing media. Damage to the material, i.e., corrosion and cavitation results. The degree of damage depends on the quality of the metals, the nature of the cooling or heat-transfer fluid, and on the external conditions of the cooling cycle. The damage to the material can be effectively combated and reduced by addition of suitable inhibitors to the functional fluids, but can be prevented entirely only in the rarest cases.
Preferred corrosion-protecting agents are benzoates, borax, nitriles, and occasionally, phosphates. The effect of such inhibitors is specific and usually is restricted to one metal group when used within a limited concentration range. As the metals are exposed to increased stress, for example due to cavitation, the inhibitors may in certain cases again lose their beneficial protective effect and cause increased damage to the materials. The damage to certain metals, for example, aluminum, can in this connection, definitely be worse than that which would otherwise occur had there been no inhibitor added at all.
Cavitation damage can be observed, due to conditions of construction, primarily on components made of cast iron and aluminum, e.g., on materials normally used in pumps. Such damage occurs predominantly in cooling systems for internal combustion engines. In addition to the water pump, endangered are the cylinder liners, parts of the crankcase, radiators and others. Accordingly, cast iron and aluminum, together with its alloys, are to be protected principally against cavitation. The conventional corrosion-protecting additives are suitable for this purpose only in exceptional cases.
In water, borate-nitrite mixtures show a cavitation-inhibiting effect with respect to gray cast iron. However, in glycol-water mixtures, this inhibitor combination results in increased erosion of aluminum. Also, corrosion inhibition with sodium benzoate and sodium nitrite or sodium nitrate permits unduly high cavitation erosions on aluminum and aluminum alloys. Both formulations moreover, are sensitive to certain chemicals, such as, for example, sodium carbonate, which are frequently utilized to set a minimum alkalinity of the coolants and to protect against superacidification.
Cavitation-inhibiting cooling liquids for diesel engines are known from DAS's [German Published Applications] 1,239,137 and 1,263,398. They contain polyalkylene glycols or polyoxyethylates of alcohols, carboxylic acids or carboxylic acid amides. Also, cavitation-inhibiting cooling fluids for internal combustion engines are known from DAS No. 1,239,138. These contain polyvinylpyrrolidone.
These prior-art additives are effective, however, only on gray cast iron, and even in this case, only when used in comparatively high concentrations (0.1-5% by weight). These additives are practically ineffective with respect to aluminum, as will be demonstrated below by comparative tests (Tables 1 and 2). This, of course, is especially disadvantageous in that a prime objective is to improve cavitation protection and corrosion protection with respect to aluminum and aluminum alloys in view of the increased usage of these light metals in engine construction.